sin = the subject's own. hans / hennes = someone else's. English his covers both.
SwedishLevel B1
Erik tvättar sin bil means Erik washes his own car: sin points back to the subject. Erik tvättar hans bil means he washes another man's car: hans points to someone else.
sin goes with en-words (sin bil), sitt with ett-words (sitt hus) and sina with plurals (sina barn). It can never be part of the subject: Hans bil är röd, not Sin bil är röd.
Examples
Hon har fått sin första tand.
She has got her first tooth.
Anna ringde sin mamma.
Anna called her (own) mum.
Hon älskar sitt jobb.
She loves her job.
De sålde sina bilar.
They sold their (own) cars.
De sålde deras bilar.
They sold their cars (somebody else's).
Han och hans fru kom sent.
He and his wife came late. (part of the subject, so hans)
Common mistake
Han älskar hans fru means he loves another man's wife. For his own wife: Han älskar sin fru. Likewise Hon tappade sina nycklar (she lost her own keys), not hennes.